Mock Observations for the CSST Mission: HSTDM--Synthetic Data Generation

TL;DR

This paper presents a practical framework for generating synthetic observations for the HSTDM backend of the CSST, targeting rigorous testing of the ground-based HSTDM data processing pipeline across Target and On-The-Fly observing modes. It combines models of instrumental instability via an Ornstein-Uhlenbeck process, space-environment effects (sunlit/shaded coupling, cosmic-ray interference, CMB), and mode-specific data-flow timelines to produce Level 0 data streams that feed Level 1 and Level 2 pipelines. The authors implement the approach in Python, demonstrate target-mode and OTF scenarios, and validate Level 1 calibration and Level 2 regridding against independent references (brightness-temperature tests and GILDAS maps), highlighting both successes and boundary-condition discrepancies. The work provides a timely tool for early pipeline verification, optimization of observation strategies, and robust data product generation ahead of ground testing and in-flight operations.

Abstract

The High Sensitivity Terahertz Detection Module (HSTDM), a key component of the backend modules on board the China Space Station Telescope (CSST), will offer great opportunities for the discovery of Terahertz Astronomy, with implications that extend well beyond China to the global astronomical community. It is imperative that the raw data collected by HSTDM undergoes meticulous calibration and processing through the HSTDM data processing pipeline (HSTDM pipeline for short) to ensure the accuracy and effectiveness of the final science data to be archived for further research. This process necessitates that the HSTDM pipeline address instrumental artifacts and effects as well as the coordination of data flow of the scheduled observing sequences under all observing modes of HSTDM within the CSST automated processing environment. As the understanding of CSST HSTDM data processing develops during the pipeline development stage, it becomes essential to assess the accuracy, the robustness and the performance of the HSTDM pipeline under all observing modes of HSTDM so that components of the HSTDM pipeline be rationally added, removed, amended or extended within the modular framework. In this paper, we develop practical simulation methods to facilitate this need. The contribution of synthetic data generation of HSTDM observation includes two parts: 1. HSTDM instrumental effect simulation based on both real testing profiles and simulated models; 2. Observing data flow generation based on HSTDM observing mode scenario. The simulation methods have been implemented and shown to be practical in testing the HSTDM pipeline during the development stage.

Figures (13)

• Figure 1: The illustration of position switch and chop load operation during HSTDM observation. The position switch operation involves telescope slewing to change the actual pointing from the source position (ON) to the reference position (OFF). The chop load operation involves the internal mirror's adjustment to change the sky path towards the internal load, which is usually a carefully engineered blackbody radiator with high emissivity and known physical temperature. This figure is an adaptation from the HIFI observing mode illustration on page 33, 36 in reference Hhifi2017.
• Figure 2: Timeline of target mode observation for HSTDM. Except for the initial instrument setup, the timeline consists of several cycles of basic operations such as integrations at the source position, integrations on the cold load during the source to OFF position slewing, and integrations at the OFF position.
• Figure 3: Schematic illustration of the OTF observation mode. The process of integrating on source map and data dumping takes place as the telescope performs a scan along a particular row within the grid. After completing each row, the telescope reverses its scanning direction to commence the scan of the subsequent row. Integration and data dumping activities are temporarily suspended during these directional transitions. Once a period has elapsed, as determined by the system's stability criteria, the mapping procedure is paused momentarily to conduct reference measurements at the reference (OFF) position.
• Figure 4: Timeline of the OTF observation for HSTDM. Except for the initial instrument setup, the timeline mainly consists of several cycles of basic operations such as integrations at source map during the scanning, a subsequent slew to a user designated OFF position, integration on cold load during the slewing, and integrations at the OFF position.
• Figure 5: Overview of the HSTDM data processing pipeline. The HSTDM data processing pipeline is a two-tiered pipeline designed for execution at ground stations. The pipeline's inputs and outputs correspond to Level 0 data, Level 1 data products, and Level 2 data products, respectively. Essential for its operation are the scheduling file and the On Orbit Calibration file, which are provided by the auxiliary systems.